The present invention relates to battery chargers, and more particularly, to battery charging apparatus that provides for battery monitoring, charge control, and battery balancing for use with an electric vehicle, and the like.
The assignee of the present invention designs, develops and manufactures battery charging systems for use in charging batteries of electric vehicles, and the like. Battery packs used in electric vehicles are typically strings of batteries, having on the order of 150 series coupled cells. The ability to balance the energy contained in each of the batteries improves their life and useful capacity.
There is a tendency for each cell within individual batteries, when connected in series, to have a different energy storage capacity. This is caused be many variables including, but not limited to, temperature, initial tolerances, material impurities, porosity, electrolyte density, surface contamination, and age. When discharging a series string of batteries (a battery pack), the amount of useful energy that can be extracted depends upon the weakest cell or battery. Battery balancing is therefore periodically required to maximize energy storage capacity of the battery pack. Reduced energy storage capacity equates to reduced battery pack life. This is due to the damage caused by "cell reversal" encountered when one cell is much different from the others in terms of its stored energy.
The classical means for balancing a battery pack is called equalization charging. This involves passing a low current through the battery pack thus charging the low cells while the fully charged cells slowly evolve gas (through electrolysis). It is done at a low current to minimize damage to "good" cells.
Balancing batteries during discharge (propulsion of an electric vehicle, for example) is especially important for situations where only quick recharging is used. Balancing is a low magnitude but long duration process, and conventional equalization charging is also a slow process. Quick recharging is unable to fully charge a battery pack due to limitations of most battery chemistries (chemical transport limit). This tends to aggravate balancing because there is no time for periodic equalization charging.
Therefore, it is an objective of the present invention to provide for battery charging apparatus that provides for battery monitoring, charge control, and battery balancing for use with an electric vehicle, and the like.